Factors influencing steam temperature variation constantly change in a modern power boiler. The principal factors driving steam temperature variation are boiler fouling, changes in load, and shifts in fuel quality. At the same time, accurate control of the final steam temperature attained in a modern power boiler is critical from both a performance and reliability perspective. A deviation of 35° to 40° F. in final steam temperature corresponds to a change of approximately one percent in turbine cycle heat rate. Off-design steam temperature can also lead to reductions in steam generator output. In addition, prolonged operation at higher-than-design steam temperatures can lead to exceeding design stress limits in boiler and turbine-cycle metal components, while prolonged operation at lower-than-design steam temperatures can result in moisture erosion damage in the final lower pressure stage.
Known control techniques to regulate final steam temperature during operation include soot blowing, attemperating water sprays injected into the steam conduit, bypassing of flue gas around heat exchangers, and tilting burners.
Known flue gas recycle is capable of directly controlling only one of the steam temperatures, while allowing inaccuracy in the control of other steam temperatures. This inaccuracy leads to penalties in thermal efficiency, power generation, and/or unit reliability. In a known flue gas recycle (FGR), flue gas is recycled upstream of the inlet of a convective pass (perhaps in a furnace itself) in a combustion system to achieve steam attemperation. This method is inherently unable to simultaneously control both the main (i.e. throttle) steam and reheat steam temperatures. Known flue gas recycle is capable of directly controlling only one of the steam temperatures, while allowing inaccuracy in the control of other steam temperatures. This inaccuracy leads to penalties in thermal efficiency, penalties in power generation, and/or penalties in unit reliability.
Attainment of design main and reheat steam temperatures is important for achieving optimal thermal efficiency, more power generation, and higher steam turbine reliability. Operating steam temperatures in conventional air/fuel boilers can be affected by factors such as changes in unit load, variability in fuel quality, and fouling of heat exchanger sections. In general, the response of main and reheat steam temperature to the various disturbances result in different magnitude deviations of steam temperature from the set point values. The challenges of maintaining independent steam temperature control in oxy/fuel boilers is further compounded by the lack of empirical data to accurately predict the heat release and fouling characteristics in this future technology. Hence, the potential exists for significant steam temperature bias (i.e. deviation from desired values) inadvertently built into the boiler design, as well as larger deviations occurring during normal operation.
Oxy/coal combustion technology (combustion technology for combusting coal with oxygen) has no industrial scale operating data upon which to predict the effects that changes in factors such as flame and gas emissivity, flame length, slagging and fouling, etc. have on boiler performance. To accommodate this, a larger range of control of steam temperature than for existing air/fuel designs is desired in order to avoid performance and reliability concerns.
What is needed is a device and method for achieving independent control of multiple steam temperatures in an oxy/fuel boiler employing flue gas recycle which, by so doing, substantially avoids the aforementioned penalties.